This invention relates generally to methods for removing bubbles from chambers, including passageway chambers, containing a liquid, and, more particularly, to a method and apparatus for removing gas bubbles from a liquid-filled chamber having medical sensory equipment such as a blood gas and chemistry analyzer.
Systems for measuring certain chemical characteristics of fluids, e.g., the concentration of certain analytes such as ions, gases and metabolites in human blood, can take the form of blood chemistry diagnostic systems integrated into infusion fluid delivery systems of the kind commonly used in hospital patient care. Such fluid delivery systems infuse nutrients, medications and the like directly into the patient at a controlled rate and in precise quantities for maximum effectiveness. Infusion fluid delivery systems are connected to a patient at an intravascular (IV) port, in which a hollow needle/catheter combination, with an exposed female luer connector, is inserted into a blood vessel of the patient and thereafter an infusion fluid is introduced into the blood vessel at a controlled rate, typically using a peristaltic pump.
Blood chemistry monitoring systems that are combined with infusion delivery systems of this kind use the IV port to periodically withdraw a blood sample back into a fluid measuring chamber along the path of the infusion fluid. Instruments in the chamber perform measurements of blood ion concentrations and the like, and then either discard the blood or reinfuse it into the patient. The system then resumes delivery of the infusion fluid. See, e.g., U.S. Pat. No. 5,431,174, incorporated herein by reference.
During preparation and/or use, small bubbles can be formed within or carried into the fluid measuring chamber of the blood chemistry monitoring system. These bubbles can attach to the walls or other structures of the chamber and affect the performance of sensors designed to measure concentrations of compounds in the fluid. Due to the strength of surface tension at the bubble-to-chamber-wall interface, such bubbles can be very difficult to remove through fluid movement because the force exerted on the bubble by the fluid movement is small compared to the surface tension holding it to the chamber surface.
Rapid fluid movement, either laminar or turbulent, has frequently been used to remove attached bubbles from chamber walls. When the force exerted by the moving fluid on the bubble is greater than the attachment force, the bubble breaks free and can be carried out of the measuring chamber. Unfortunately, the fluid velocity required to achieve this force can be considerable and often exceeds the velocity required for normal operation of the chamber to measure fluids in a sequential manner. Furthermore, any increase in fluid pumping rate usually increases the local pressure at a bubble, reducing its size and cross sectional area, reducing the resultant force from the moving fluid. Thus, fluid pumps must be sized considerably larger than would otherwise be required, leading to increased size, complexity, and cost.
Other methods of bubble removal have involved improvements in the design of the measuring chamber shape for high velocity flow at relatively low fluid pumping rates. Also, use of materials or chamber surface treatments that reduce the surface tension strength, thus enabling lower pumping rates to dislodge the bubble, have been shown to be effective. However, pumps, fluid chambers and pathways of a measurement system for intravenous use must be manufactured and maintained in a sterile and non-toxic condition. These restrictive design requirements limit the choice of materials that can be used in the fluid path, the design and performance of the fluid pumping system, and the complexity of the measurement chamber. Furthermore, sensor size, shape, and design details required for cost effective manufacture and use can preclude these otherwise desirable chamber design characteristics.
This is particularly true for apparatus designed to perform the fluid and body fluid measurements outside of the laboratory, with analyzers that have been designed to be portable and can be operated by personnel with less training. Often, compromises in pumping system performance and measurement system cost and complexity are required to meet such portability, operating cost, and ease-of-use requirements.
In dealing with a separate problem, it is known that the slow removal of a fluid-aspiration-and-dispense tip from a fluid will remove small droplets of fluid that might otherwise cling to the exterior surface of the tip. This technique is often referred to as a “pool wipe.” This technique is typically limited to external probes used to aspirate and dispense fluids from open vessels.
Some or all of these difficulties apply generally to other apparatus suffering from bubbles in chambers, including passageway chambers, containing a fluid. Thus, while the present invention can be understood as a method for removing gas bubbles from a liquid-filled chamber having medical sensory equipment such as a blood gas and chemistry analyzer, it should also be understood more generally as method of removing bubbles in chambers, including passageway chambers, containing fluid.
Thus, there has existed a definite need for a method and/or an apparatus to remove small bubbles from a chamber. Such an invention will preferably provide for sensor measurements to be performed with minimum risk of error due to bubbles. The present invention satisfies these and other needs, and provides further related advantages.